The present invention relates to a perpendicular magnetic storage medium and more particularly to a perpendicular magnetic storage medium used in a form of a card, a tape, a disc or the like, wherein there is formed a perpendicular magnetic anisotropic film made of (1) a sub-oxide of a metal comprising Fe and Co, or (2) a sub-oxide of a metal comprising Fe, Co and at least one metal other than the above two, which is suitable for high density recording.
Hitherto, a perpendicular magnetic storage medium is effectively used for a high-density mangetic storage. A magnetic storage medium used for such a purpose utilizes a magnetic thin film which has an easy magnetization axis being perpendicular to the plane of the film. As a magnetic film for the above use, there have been used or researched a thin film of Co-Cr (Co-Cr alloy), Fe.sub.3 O.sub.4 or O.sub.s -.gamma.Fe.sub.2 O.sub.3 produced by sputtering method or vacuum deposition method, a thin film of barium-ferrite produced by coating method or sputtering method, and the like.
Further, in order to improve the recording/reproducing sensitivity of a magnetic thin film, it has been proposed to apply a double layer film construction, wherein a soft magnetic layer is provided between the substrate and the perpendicular magnetic anisotropic film, in a perpendicular magnetic storage medium. The above proposal is given in Japanese Examined Patent Publication No. 91/1983. For instance, a perpendicular magnetic storage medium with the double layer film construction which uses a perpendicular magnetic anisotropic film of Co-Cr has a ten times higher recording/reproducing sensitivity than that uses the same film but with single layer construction.
However, the above conventional perpendicular magnetic storage mediums have drawbacks as described hereinafter.
The magnetic thin film of Co-Cr alloy used in the above conventional medium is required to have a structure close to a single crystal. Therefore, a substrate, on which the magnetic thin film is formed, should be heated to higher than 100.degree. C. or frequently higher than 200.degree. C. during the deposition thereon. This requires the use of heat-resistant substrate and consequently increases the production cost. Further, a film of metal has an intrinsic drawback that it is apt to be worn out.
The magnetic thin film of the metal oxide such as Fe.sub.3 O.sub.4 and O.sub.s -.gamma.Fe.sub.2 O.sub.3 is hard and has a high wear resistance. But, since a substrate for the magnetic thin film of Fe.sub.3 O.sub.4 or O.sub.s -.gamma.Fe.sub.2 O.sub.3 is also required to be heated to higher than 250.degree. C. during the deposition thereon, the production cost is increased like that of the above Co-Cr alloy film. Further, the metal oxide such as O.sub.s -.gamma.Fe.sub.2 O.sub.3 or the like used for the magnetic thin film is sometimes required reduction process. Still further, the perpendicular magnetic storage medium using these magnetic thin films of the metal oxide has a drawback that the saturation magnetization (Ms) is low and therefore there cannot be obtained mediums having high recording/reproducing sensitivity.
The thin film of barium-ferrite obtained by coating method for the perpendicular magnetic storage medium requires to provide barium-ferrite powder having a uniform particle diameter of about 0.1 .mu.m during the forming of the film. Accordingly, the production cost is increased. Further, because a binder is required to be added to form a film, the content of barium-ferrite in the film is lowered. This causes a low saturation magnetization (Ms) of the magnetic thin film, and consequently lowers the performance of the magnetic storage medium.
The thin film of barium-ferrite obtained by sputtering method has a higher saturation magnetization than the film by coating method, but a substrate therefor should be heated to about 500.degree. C. Accordingly, the substrate must be highly heat resistant, and the use of an inexpensive plastic substrate is not allowed.
The perpendicular magnetic storage medium having a double layer film construction, wherein a soft magnetic film is provided between the substrate and a perpendicular magnetic anisotropic film so that the recording/reproducing sensitivity in recording an information into the magnetic thin film and reproducing it out of the film can be improved, also has a drawback, because the specifications of the two films often restrict each other due to each individual characteristics as a crystal. For example, in a perpendicular magnetic anisotropic film of Co-Cr alloy, the crystal axis hcp &lt;001&gt; is required to be oriented perpendicuarly to the film surface, and for this purpose, the soft magnetic film should be strictly specified in a kind of a material, a configuration of a crystal, a lattice constant and a degree of orientation.
As an effective way to remove the above drawbacks, there is proposed a perpendicular magnetic anisotropic film of a sub-oxide of Co. This film can be formed at a low sbstrate temperature, so the use of a low heat resistant and inexpensive film substrate is allowed. And this perpendicular magnetic anisotropic film has a high perpendicular magnetic anisotropy, whereby a perpendicular magnetic anisotropic film having a high saturation magnetization (Ms) can be obtained. But, there is a problem that a perpendicular magnetic anisotropic film of a sub-oxide of Co does not have a high recording/reproducing sensitivity.
Further, we inventors have proposed a perpendicular magnetic anisotropic film of a sub-oxide of Fe. This film can also be formed at a low substrate temperature. But the film of a sub-oxide of Fe does not have either a sufficient perpendicular magnetic anisotropy, or a large saturation magnetization (Ms).
Still further, there has been proposed, a perpendicular magnetic anisotropic film of a sub-oxide of Co--Fe, or Co--Fe--Ni. This film can be produced at a low substrate temperature. But the recording/reproducing sensitivity thereof is not high.
It should be noted that the perpendicular magnetic anisotropic film of the above sub-oxides have advantages of high flexibility due to metals contained therein and high wear resistance.
The present invention was made to solve the above mentioned problem existing in the conventional perpendicular magnetic anisotropic films themselves, and the problem that a perpendicular magnetic anisotropic film and a soft magnetic layer are mutually restricted, and an object of the present invention is to provide a perpendicular magnetic storage medium having a perpendicular magnetic anisotropic film, which can be produced easily and economically at a low substrate temperature, and also which has high wear resistance, high oxidation resistance, high recording/reproducing sensitivity and high recording density, and further which requires less reciprocal restrictions with a soft magnetic layer in applying a double layer construction wherein the soft magnetic layer is used.